1 files changed, 319 insertions, 0 deletions
diff --git a/contrib/pgstattuple/pgstatapprox.c b/contrib/pgstattuple/pgstatapprox.c
new file mode 100644
index 0000000..15ddc32
--- /dev/null
+++ b/contrib/pgstattuple/pgstatapprox.c
@@ -0,0 +1,319 @@
+/*-------------------------------------------------------------------------
+ *
+ * pgstatapprox.c
+ *		  Bloat estimation functions
+ *
+ * Copyright (c) 2014-2022, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		  contrib/pgstattuple/pgstatapprox.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/heapam.h"
+#include "access/htup_details.h"
+#include "access/multixact.h"
+#include "access/relation.h"
+#include "access/transam.h"
+#include "access/visibilitymap.h"
+#include "access/xact.h"
+#include "catalog/namespace.h"
+#include "catalog/pg_am_d.h"
+#include "commands/vacuum.h"
+#include "funcapi.h"
+#include "miscadmin.h"
+#include "storage/bufmgr.h"
+#include "storage/freespace.h"
+#include "storage/lmgr.h"
+#include "storage/procarray.h"
+#include "utils/builtins.h"
+
+PG_FUNCTION_INFO_V1(pgstattuple_approx);
+PG_FUNCTION_INFO_V1(pgstattuple_approx_v1_5);
+
+Datum		pgstattuple_approx_internal(Oid relid, FunctionCallInfo fcinfo);
+
+typedef struct output_type
+{
+	uint64		table_len;
+	uint64		scanned_percent;
+	uint64		tuple_count;
+	uint64		tuple_len;
+	double		tuple_percent;
+	uint64		dead_tuple_count;
+	uint64		dead_tuple_len;
+	double		dead_tuple_percent;
+	uint64		free_space;
+	double		free_percent;
+} output_type;
+
+#define NUM_OUTPUT_COLUMNS 10
+
+/*
+ * This function takes an already open relation and scans its pages,
+ * skipping those that have the corresponding visibility map bit set.
+ * For pages we skip, we find the free space from the free space map
+ * and approximate tuple_len on that basis. For the others, we count
+ * the exact number of dead tuples etc.
+ *
+ * This scan is loosely based on vacuumlazy.c:lazy_scan_heap(), but
+ * we do not try to avoid skipping single pages.
+ */
+static void
+statapprox_heap(Relation rel, output_type *stat)
+{
+	BlockNumber scanned,
+				nblocks,
+				blkno;
+	Buffer		vmbuffer = InvalidBuffer;
+	BufferAccessStrategy bstrategy;
+	TransactionId OldestXmin;
+
+	OldestXmin = GetOldestNonRemovableTransactionId(rel);
+	bstrategy = GetAccessStrategy(BAS_BULKREAD);
+
+	nblocks = RelationGetNumberOfBlocks(rel);
+	scanned = 0;
+
+	for (blkno = 0; blkno < nblocks; blkno++)
+	{
+		Buffer		buf;
+		Page		page;
+		OffsetNumber offnum,
+					maxoff;
+		Size		freespace;
+
+		CHECK_FOR_INTERRUPTS();
+
+		/*
+		 * If the page has only visible tuples, then we can find out the free
+		 * space from the FSM and move on.
+		 */
+		if (VM_ALL_VISIBLE(rel, blkno, &vmbuffer))
+		{
+			freespace = GetRecordedFreeSpace(rel, blkno);
+			stat->tuple_len += BLCKSZ - freespace;
+			stat->free_space += freespace;
+			continue;
+		}
+
+		buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno,
+								 RBM_NORMAL, bstrategy);
+
+		LockBuffer(buf, BUFFER_LOCK_SHARE);
+
+		page = BufferGetPage(buf);
+
+		/*
+		 * It's not safe to call PageGetHeapFreeSpace() on new pages, so we
+		 * treat them as being free space for our purposes.
+		 */
+		if (!PageIsNew(page))
+			stat->free_space += PageGetHeapFreeSpace(page);
+		else
+			stat->free_space += BLCKSZ - SizeOfPageHeaderData;
+
+		/* We may count the page as scanned even if it's new/empty */
+		scanned++;
+
+		if (PageIsNew(page) || PageIsEmpty(page))
+		{
+			UnlockReleaseBuffer(buf);
+			continue;
+		}
+
+		/*
+		 * Look at each tuple on the page and decide whether it's live or
+		 * dead, then count it and its size. Unlike lazy_scan_heap, we can
+		 * afford to ignore problems and special cases.
+		 */
+		maxoff = PageGetMaxOffsetNumber(page);
+
+		for (offnum = FirstOffsetNumber;
+			 offnum <= maxoff;
+			 offnum = OffsetNumberNext(offnum))
+		{
+			ItemId		itemid;
+			HeapTupleData tuple;
+
+			itemid = PageGetItemId(page, offnum);
+
+			if (!ItemIdIsUsed(itemid) || ItemIdIsRedirected(itemid) ||
+				ItemIdIsDead(itemid))
+			{
+				continue;
+			}
+
+			Assert(ItemIdIsNormal(itemid));
+
+			ItemPointerSet(&(tuple.t_self), blkno, offnum);
+
+			tuple.t_data = (HeapTupleHeader) PageGetItem(page, itemid);
+			tuple.t_len = ItemIdGetLength(itemid);
+			tuple.t_tableOid = RelationGetRelid(rel);
+
+			/*
+			 * We follow VACUUM's lead in counting INSERT_IN_PROGRESS tuples
+			 * as "dead" while DELETE_IN_PROGRESS tuples are "live".  We don't
+			 * bother distinguishing tuples inserted/deleted by our own
+			 * transaction.
+			 */
+			switch (HeapTupleSatisfiesVacuum(&tuple, OldestXmin, buf))
+			{
+				case HEAPTUPLE_LIVE:
+				case HEAPTUPLE_DELETE_IN_PROGRESS:
+					stat->tuple_len += tuple.t_len;
+					stat->tuple_count++;
+					break;
+				case HEAPTUPLE_DEAD:
+				case HEAPTUPLE_RECENTLY_DEAD:
+				case HEAPTUPLE_INSERT_IN_PROGRESS:
+					stat->dead_tuple_len += tuple.t_len;
+					stat->dead_tuple_count++;
+					break;
+				default:
+					elog(ERROR, "unexpected HeapTupleSatisfiesVacuum result");
+					break;
+			}
+		}
+
+		UnlockReleaseBuffer(buf);
+	}
+
+	stat->table_len = (uint64) nblocks * BLCKSZ;
+
+	/*
+	 * We don't know how many tuples are in the pages we didn't scan, so
+	 * extrapolate the live-tuple count to the whole table in the same way
+	 * that VACUUM does.  (Like VACUUM, we're not taking a random sample, so
+	 * just extrapolating linearly seems unsafe.)  There should be no dead
+	 * tuples in all-visible pages, so no correction is needed for that, and
+	 * we already accounted for the space in those pages, too.
+	 */
+	stat->tuple_count = vac_estimate_reltuples(rel, nblocks, scanned,
+											   stat->tuple_count);
+
+	/* It's not clear if we could get -1 here, but be safe. */
+	stat->tuple_count = Max(stat->tuple_count, 0);
+
+	/*
+	 * Calculate percentages if the relation has one or more pages.
+	 */
+	if (nblocks != 0)
+	{
+		stat->scanned_percent = 100 * scanned / nblocks;
+		stat->tuple_percent = 100.0 * stat->tuple_len / stat->table_len;
+		stat->dead_tuple_percent = 100.0 * stat->dead_tuple_len / stat->table_len;
+		stat->free_percent = 100.0 * stat->free_space / stat->table_len;
+	}
+
+	if (BufferIsValid(vmbuffer))
+	{
+		ReleaseBuffer(vmbuffer);
+		vmbuffer = InvalidBuffer;
+	}
+}
+
+/*
+ * Returns estimated live/dead tuple statistics for the given relid.
+ *
+ * The superuser() check here must be kept as the library might be upgraded
+ * without the extension being upgraded, meaning that in pre-1.5 installations
+ * these functions could be called by any user.
+ */
+Datum
+pgstattuple_approx(PG_FUNCTION_ARGS)
+{
+	Oid			relid = PG_GETARG_OID(0);
+
+	if (!superuser())
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("must be superuser to use pgstattuple functions")));
+
+	PG_RETURN_DATUM(pgstattuple_approx_internal(relid, fcinfo));
+}
+
+/*
+ * As of pgstattuple version 1.5, we no longer need to check if the user
+ * is a superuser because we REVOKE EXECUTE on the SQL function from PUBLIC.
+ * Users can then grant access to it based on their policies.
+ *
+ * Otherwise identical to pgstattuple_approx (above).
+ */
+Datum
+pgstattuple_approx_v1_5(PG_FUNCTION_ARGS)
+{
+	Oid			relid = PG_GETARG_OID(0);
+
+	PG_RETURN_DATUM(pgstattuple_approx_internal(relid, fcinfo));
+}
+
+Datum
+pgstattuple_approx_internal(Oid relid, FunctionCallInfo fcinfo)
+{
+	Relation	rel;
+	output_type stat = {0};
+	TupleDesc	tupdesc;
+	bool		nulls[NUM_OUTPUT_COLUMNS];
+	Datum		values[NUM_OUTPUT_COLUMNS];
+	HeapTuple	ret;
+	int			i = 0;
+
+	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+		elog(ERROR, "return type must be a row type");
+
+	if (tupdesc->natts != NUM_OUTPUT_COLUMNS)
+		elog(ERROR, "incorrect number of output arguments");
+
+	rel = relation_open(relid, AccessShareLock);
+
+	/*
+	 * Reject attempts to read non-local temporary relations; we would be
+	 * likely to get wrong data since we have no visibility into the owning
+	 * session's local buffers.
+	 */
+	if (RELATION_IS_OTHER_TEMP(rel))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot access temporary tables of other sessions")));
+
+	/*
+	 * We support only relation kinds with a visibility map and a free space
+	 * map.
+	 */
+	if (!(rel->rd_rel->relkind == RELKIND_RELATION ||
+		  rel->rd_rel->relkind == RELKIND_MATVIEW ||
+		  rel->rd_rel->relkind == RELKIND_TOASTVALUE))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("relation \"%s\" is of wrong relation kind",
+						RelationGetRelationName(rel)),
+				 errdetail_relkind_not_supported(rel->rd_rel->relkind)));
+
+	if (rel->rd_rel->relam != HEAP_TABLE_AM_OID)
+		ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						errmsg("only heap AM is supported")));
+
+	statapprox_heap(rel, &stat);
+
+	relation_close(rel, AccessShareLock);
+
+	memset(nulls, 0, sizeof(nulls));
+
+	values[i++] = Int64GetDatum(stat.table_len);
+	values[i++] = Float8GetDatum(stat.scanned_percent);
+	values[i++] = Int64GetDatum(stat.tuple_count);
+	values[i++] = Int64GetDatum(stat.tuple_len);
+	values[i++] = Float8GetDatum(stat.tuple_percent);
+	values[i++] = Int64GetDatum(stat.dead_tuple_count);
+	values[i++] = Int64GetDatum(stat.dead_tuple_len);
+	values[i++] = Float8GetDatum(stat.dead_tuple_percent);
+	values[i++] = Int64GetDatum(stat.free_space);
+	values[i++] = Float8GetDatum(stat.free_percent);
+
+	ret = heap_form_tuple(tupdesc, values, nulls);
+	return HeapTupleGetDatum(ret);
+}